Double metal cyanide (DMC) complexes are highly active catalysts for preparing polyether polyols by epoxide polymerization. The catalysts enable the preparation of polyether polyols having narrow molecular weight distributions and very low unsaturation (low monol content) even at high molecular weights. Recent improvements have resulted in DMC catalysts that have exceptional activity. See, for example, U.S. Pat. No. 5,470,813.
While DMC catalysts have been known since the 1960s, commercialization of polyols made from these catalysts is a recent phenomenon, and most commercial polyether polyols are still produced with potassium hydroxide. One reason for the delayed commercial availability of DMC polyols is that conventional polyol starters, e.g., water, propylene glycol, glycerin, trimethylolpropane, and the like, initiate DMC-catalyzed epoxide polymerizations sluggishly (if at all), particularly in the typical batch polyol preparation process. Typically, the polyol starter and DMC catalyst are charged to a reactor and heated with a small amount of epoxide, the catalyst becomes active, and the remaining epoxide is added continuously to the reactor to complete the polymerization.
In a typical batch process for making polyols using either KOH or a DMC catalyst, all of the polyol starter is charged initially to the reactor. When KOH is used as the catalyst, it is well understood by those skilled in the art that continuous addition of the starter (usually a low molecular weight polyol such as glycerin or propylene glycol) with the epoxide will produce polyols having broader molecular weight distributions compared with products made by charging all of the starter initially. This is true because the rate of alkoxylation with KOH is substantially independent of polyol molecular weight. If low molecular weight species are constantly being introduced, the molecular weight distribution will broaden.
Generally, polyols having broad molecular weight distributions are undesirable because they have relatively high viscosities, which can adversely impact processability in polyurethane formulations, particularly when prepolymers are made from the polyols. In addition, polyols with narrow molecular weight distributions generally give polyurethanes with better physical properties.
Those skilled in the art have assumed that continuous addition of a starter in a DMC-catalyzed polyol synthesis would also produce polyols having relatively broad molecular weight distributions. Consequently, the DMC polyol synthesis art teaches almost exclusively to charge all of the starter to the reactor initially, and to add the epoxide continuously during the polymerization.
One exception is U.S. Pat. No. 3,404,109 (Milgrom). Milgrom teaches a small-scale process for making a polyether diol using a DMC catalyst and water as a starter. Milgrom teaches to charge a beverage bottle with DMC catalyst, all of the epoxide to be used, and water, and to heat the capped bottle and contents to polymerize the epoxide. Milgrom teaches (column 7) that "w!hen large amounts of water are employed to yield low molecular weight telomers, it is preferred to add the water incrementally because large amounts of water decrease the rate of telomerization." Incremental addition of the starter (water) is used to give a "practical" rate of reaction. Thus, Milgrom charges all of the epoxide to the reactor initially, but adds the starter incrementally.
Interestingly, Milgrom also teaches that incremental addition of water "can also be employed to give telomers of a broader molecular weight distribution than those possible where all of the water is added at the beginning of the reaction." In other words, the result expected from a DMC-catalyzed process is the same as the result obtained with a KOH-catalyzed process: continuous or incremental addition of starter should give polyols with broad molecular weight distributions. Thus, a skilled person understands from Milgrom that incremental addition of a starter to a DMC-catalyzed epoxide polymerization will produce polyols having a broader molecular weight distributions than would be obtained if all of the starter were charged initially.
Heuvelsland (U.S. Pat. No. 5,114,619) teaches a process for making polyether polyols that involves continuous addition of water and epoxide to a reaction mixture containing a barium or strontium oxide or hydroxide catalyst. A DMC-catalyzed process is not disclosed. Heuvelsland's process produces polyols with reduced unsaturation. The impact of continuous water addition in the presence of barium or strontium catalysts on polyol molecular weight distribution is not discussed. Heuvelsland notes that, unlike water, continuous addition of low molecular weight diols, triols, and polyoxyalkylene glycols does not reduce polyol unsaturation. In addition, substitution of KOH for the barium or strontium catalyst does not yield the improvement.
Because conventional polyol starters initiate so slowly with DMC catalysts, polyol starters of higher molecular weight (e.g., 400-700 mol. wt. propoxylated glycerin) are commonly used. These higher molecular weight polyol starters are preferably eliminated because they must be synthesized separately (e.g., from glycerin, propylene oxide, and KOH) using a dedicated reactor. In addition, the KOH catalyst must be removed from a starter polyol before it is used as an initiator for a DMC-catalyzed polyol preparation because even traces of basic substances often deactivate DMC catalysts. Thus, a conventional KOH polyol unit having refining capability is needed just to make a starter polyol that can be used productively with a DMC catalyst. A process that allows a DMC catalyst to be used with a conventional starter such as propylene glycol or glycerin would be valuable.
The unusually high reactivity of DMC catalysts presents another challenge to polyol manufacturers: reactor fouling. Sticky polyol gels tend to form in reactors using DMC catalysts, and these gels tend to accumulate over time, fouling the reactor and eventually forcing a shutdown. The gels, which are not observed in a conventional KOH-catalyzed polyol synthesis, are preferably eliminated.
One consequence of charging all of the starter initially as in a typical batch polyether polyol synthesis is that reactors must often be used inefficiently. For example, to make a 4000 mol. wt. polyoxypropylene diol (4K diol) from a 2000 mol. wt. polyoxypropylene diol (2K diol) "starter," the reactor is almost half full at the start of the reaction; to make 50 gallons of product, we would start with 25 gallons of 2K diol starter. A valuable process would overcome such "build ratio" limitations, and would permit efficient use of reactors regardless of the molecular weight of the starter or the product sought. For example, it would be valuable to have the option to charge our 50 gallon reactor with only 5 gallons of 2K diol starter, and still make 50 gallons of 4K diol product.
In addition to the process challenges of DMC catalysis, commercial acceptance of DMC-catalyzed polyols has been hindered by the variability of polyol processing and performance, particularly in the production of flexible and molded polyurethane foams. DMC-catalyzed polyols usually cannot be "dropped into" foam formulations designed for KOH-catalyzed polyols because the polyols do not process equivalently. DMC-catalyzed polyols often give too much or too little foam stability. Batch-to-batch variability in the polyols makes foam formulation unpredictable. The cause of this unpredictability in foam formulation with DMC-catalyzed polyols has not been well understood, and consistent results have remained elusive.
An improved process for making DMC-catalyzed polyols is needed. Particularly needed is a process that eliminates the need to separately synthesize a polyol starter by KOH catalysis, and enables the use of simple starters such as water, propylene glycol, and glycerin. A valuable process would eliminate the problem of reactor fouling by polyol gels, would make efficient use of reactors, and would overcome build-ratio limitations. Preferably, the process would give polyether polyols having relatively narrow molecular weight distributions, since these polyols are more easily processed and give polyurethanes with good physical properties. Also needed are polyols that process and perform more consistently in polyurethane formulations, especially flexible and molded foams.